This is a very good question because there is no "right" answer. In fact, the end of Under the Feet of Jesus is meant to be ambiguous.

At the end of Under the Feet of Jesus, Alejo is taken to the hospital by Estrella because he has been sprayed with insecticide while he was picking apples to sell. As a result of this (or perhaps I should say "in addition to this"), Alejo's sickness gets so much worse! When they first take him to the doctor, the nurse up front takes the money, but doesn't allow them to see the doctor. Estrella explodes. They need the money for gas to get to the hospital! So, Estrella breaks the glass and gets the money back. At the hospital, we are not told much about Alejo's condition, only that Estrella took him there, that Alejo was admitted, and it isn't long before Estrella is seen leaving the hospital in an almost religious/holy fashion:

[Estrella] stepped forward and the glass doors split open before her as if obeying her command. ... Estrella parted the doors like a sea of glass and walked through. ... [Later, Estrella stands on the roof] as immobile as an angel standing on the verge of faith.”

Believe it or not, due to the religious imagery, it is Estrella's transformation that is more important than the fate of Alejo. Why is this? Because Estrella has finally learned the words of wisdom from earlier in the novel:

If we don’t take care of each other, who would take care of us? ... We have to look out for our own.

In conclusion, remember that the ending isn't clear (and it isn't SUPPOSED to be clear). I think of it just like the ending of the movie Titanic. Remember when the old Rose goes to sleep and suddenly we see all of the people on the Titanic dressed in white, smiling at her, clapping, etc. What now is the young Rose kisses Jack and then it fades into a white light on the ceiling. Did Rose die? Is the old Rose dreaming? We don't know. We are not supposed to. We are supposed to make our own decision. Same here.